Field of the invention
The present invention relates to a focus adjustment technique in an image capturing apparatus such as a digital camera or a video camera.
Description of the Related Art
Conventionally, in image capturing apparatuses having a mode for performing image capture of a starlit sky, generally, the focus lens is moved to the infinity focal position of the imaging optical system and an image is then captured, using the fact that stars exist at an infinite distance, rather than adopting the final in-focus position obtained by autofocus. For example, Japanese Patent Laid-Open No. 4-15629 discloses a technique for, when performing image capture of stars, setting the image capture lens at the infinity focal position under a predetermined exposure condition, and starting the exposure operation when a predetermined time has elapsed after operating the release button.
However, in the technique disclosed in Japanese Patent Laid-Open No. 4-15629 above, the focus lens is moved to the infinity focal position set by design, and thus there are cases where the infinity focal position set by design shifts due to change over time, change in environmental temperature, camera orientation change and the like. Therefore, there is an issue that image capture is not necessarily performed in an in-focus state.
In order to solve this, it is effective to perform focus adjustment by an AF (autofocus) function. Conventionally, as an AF system, a contrast AF system for performing focus detection by extracting specific frequency components of imaging signals (so-called band pass filter processing) and performing contrast evaluation has been widely used.
However, in automatic focus adjustment based on a contrast AF system, focus detection needs to be performed using the contrast evaluation values of a plurality of frames. In that case, if the entire image is dark as with a starlit sky, and stars, which are to be a subject, are small, the exposure time for one frame needs to be extended in order to obtain a necessary contrast, and thus there is an issue that focus adjustment takes time.
Moreover, in automatic focus adjustment based on a contrast AF system, the peak of the contrast evaluation values is detected from the contrast evaluation values of a plurality of frames, and thus the peak of the contrast evaluation value needs to be passed. Therefore, there is an issue that in moving image capture and the like, reciprocal motion of the focus lens appears in images, and the quality of the images is reduced.
In view of this, as an AF system, Japanese Patent Laid-Open No. 2008-52009 discloses an apparatus that performs focus detection based on pupil division using a two-dimensional image sensor in which each of the pixels has a microlens formed thereon. In this apparatus, a configuration is adopted in which the photoelectric conversion portion of each of the pixels that constitute the image sensor is divided into a plurality of sections, and the divided photoelectric conversion portions receive, via the microlenses, light flux that passed through different areas of the pupil of the image capture lens. Phase difference calculation for calculating a phase difference that is a deviation amount is performed for a pair of output signals of the photoelectric conversion portions that receive the light flux that passed through the different areas of the pupil of this image capture lens, and a defocus amount (Def) can then be calculated from the phase difference.
In addition, Japanese Patent Laid-Open No. 2013-11783 discloses a focus detection apparatus that changes a shift amount in phase difference detection AF in accordance with an aperture value. In this focus detection apparatus, the higher the aperture value is, the more the light flux that enters is restricted, and the smaller the distance between the centroids of two images used in phase difference detection AF becomes, and thus the shift amount is reduced. Accordingly, the time required to calculate the defocus amount (Def) can be shortened.
However, in image capture of a starlit sky, the entire sky is dark, and thus there is desire to add pixel signals in the readout direction in order to improve the S/N ratio. However, on the other hand, the sizes of the stars in the sky are extremely small, and if the pixel signals are added, the resolution decreases, making it difficult to detect the stars. Therefore, in order to improve the S/N ratio and not reduce the resolution, it is necessary to perform image capture with a bright f-number (in a state where the diaphragm is open) without adding the pixel signals in the readout direction. However, in the method described in Japanese Patent Laid-Open No. 2013-11783, if the diaphragm is opened, the shift amount between two images increases, and it takes a long time to calculate the defocus amount (Def).